This invention relates to apparatus for determining the location of an edge of a document, and in particular, but not exclusively, to a position sensing means for use in a banknote validator.
Various types of banknote validator are known. One such type is a banknote validator using an optical sensing head which illuminates a banknote and senses the spectral response of the banknote to determine whether the banknote paper and/or the printing inks are genuine. Such validators are described in U. S. Pat. Nos. 5,140,166 and 5,381,021. A banknote passing through a transport mechanism is sensed using an optical head. Because the transport mechanism pre-aligns the banknote with a lateral edge before the banknote is sensed, the lateral position of the banknote is known. The position of the banknote as it moves through the transport mechanism can be determined simply by sensing the leading and trailing edges of the banknote.
However, banknote validators using such a pre-alignment mechanism are able to process banknotes only at a limited rate due to the need for mechanical pre-alignment of banknotes entering the transport mechanism.
British patent application GB 2029007 describes a banknote validator incorporating a width and length detection arrangement. The banknote is passed through a light curtain consisting of a number of photosensors arranged in a line across a transport path for banknotes. The leading and trailing edges of the banknotes are sensed, the number of intervening scan pulses being counted in order to determine the banknote length. Meanwhile, the width of the banknote is determined by counting the number of darkened photodiodes after the banknote enters the light curtain. A serial diode signal (SDS), which is read out from a shift register, exhibits logic 1 and logic 0 states, and the number of 1s counted is a measure of the width of the banknote. The banknote is always prealigned, as the lower edge of the banknote moves on a sliding surface during movement through the light curtain.
In an arrangement described in U. S. Pat. No. 4,855,607, an array of photodiodes is used in conjunction with a deskewing mechanism for sensing the skew of a moving mail piece and then deskewing the mail piece. In an example given of a transmissive photosensor, the skew of the moving mail piece is detected by changes in the number and location of the individual photosensors which are occluded during each sampling period as the mail piece moves into the apparatus. The speed of asymmetric drive motors are varied according to the sensed skew, in order to align the mail niece with a transfer writing head.
In a further arrangement described in Texas Instruments product brochure "Showcase" issue 20, at page 9, a line imager is described which is suitable for the sensing of banknotes. The line imager is a charge mode imager, with the analog voltage per pixel proportional to the product of the light intensity on the pixel and the exposure time. The analog output voltages are shifted out serially, and 16 grey scales of output are possible. It is stated to be suitable for reading black and white documents, and measuring marks or edges.